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Book Chapter

3-D EM Inversion to the Limit

By
James Macnae
James Macnae
1Cooperative Research Centre for Australian Mineral Exploration Technologies, Macquarie University, New South Wales 2109, Australia.
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Andrew King
Andrew King
1Cooperative Research Centre for Australian Mineral Exploration Technologies, Macquarie University, New South Wales 2109, Australia.
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Ned Stolz
Ned Stolz
2Western Mining Ltd., P.O. Box 22, Leinster, Western Australia 6437, Australia.
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Philip Klinkert
Philip Klinkert
3Geophysics Department, Anglo American Corporation, 45 Main St., Johannesburg, South Africa.
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Published:
January 01, 1999

Summary

In a typical day of mineral exploration, an airborne electromagnetic (EM) system may cover 500 km of line, recording data at 10-m spacings. Wide-band data at the 50 000 stations then is interpreted to pick areas worthy of follow-up. In Australia, where conductive regolith is common, there may be 500 local anomalies in this data set. The first task, therefore, is to classify these anomalies by geometry and conductivity. Reduction of the original EM data at each station to only two parameters—the estimated free-space inductive and resistive limits—greatly reduces the complexity of the interpretation problem. The reduction involves stripping and correcting for background host effects. The inductive limit is a function of target geometry alone, whereas the resistive limit is linearly related to target conductivity for an isolated body, but also includes geometric effects. The calculation of forward and inverse 3-D conductivity models using these simple limits is possible in seconds on a fast personal computer. This allows for automated and interactive classification of the sources of anomalies. The results also can provide starting models for more careful inversion or modeling. The major limitation at present is that the method does not model current gathering, which takes place when conductors are in contact.

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Figures & Tables

Contents

Geophysical Developments Series

Three-Dimensional Electromagnetics

Society of Exploration Geophysicists
Volume
7
ISBN electronic:
9781560802154
Publication date:
January 01, 1999

GeoRef

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